Telewriting apparatus



May 14, 1963 R. E. GRAHAM TELEWRITING APPARATUS 2 Sheets-Sheet 1 FiledSept. 9. 1960 May 14, 1963 R. GRAHAM 3,089,918

TELEWRITING APPARATUS Filed Sept. 9. 1960 2 Sheets-Sheet 2 CENTER 0FZ5/N auf Cp/ Fla 2 VERT/CAL PHoTocELL DEFLECT/o/v c/RCULAR Locus or dBEAM /v L? /c HoR/zo/vml. DEA-ECTION a C05 cot F/G. 3 I i l l l i I I fI" f4.

/L-l I F /6 4 /ELEcrR/c LAVA-R RES/sT/VE F/LM @/g 1 [l] a CLPUEMODULATOR /a \/9 METAL l 30 STYLUS l/ l TRACK/Nc: U/v/r I 1/A/l/EA/TOR R. E. GRAHAM A 7` TURN United States Patent O 3,089,918TELEWRITING APPARATUS Robert E. Graham, Chatham Township, Morris County,

NJ., assignor to Bell Telephone Laboratories, Incorporated, New York,N.Y., a corporation of New York Filed Sept. 9, 1960, Ser. No. 54,989 12Claims. (Cl. 178-19) This invention relates to the electricaltransmission of information pertaining to graphic material and, moreparticularly, to apparatus for generating electrical signals that areindicative of the position of a moving stylus upon the face of a writingtablet. It has for an important object the elimination of mechanicalrestraints that fetter the free movement of tbs stylus.

Various types of telewn'ting devices for indicating the X and Yco-ordinates of a moving stylus adjacent to the plane formed by awriting tablet are well known. Perhaps the most common of these involvesthe use of pantographic arrangements in which the stylus -is coupled to,and thus restrained by, a series of mechanical linkages, the relativemovement of which eiects a change, for example, in the resistance of avariable resistor to which the linkages `are connected. Electr-icalsignals indicative of the position of the stylus Vare obtained byutilizing the variable resistor in an appropriate electrical circuit.General-ly, however, the use of such pantographic arrangements intelewn'ting devices has been deemed undesirable, since the mechanicallinkages associated therewith impede the completely free movement of thestylu-s and prevent a person who is using the device from writing withhis normal and natural style and speed.

Telewriting devices are known, of course, in which the above-mentionedpantographic apparatus is eliminated. 'Ihese generally utilizeunrestrained conductive styli in conjunction with resistive surfaces asthe writing tablets.

Typically, the resistive surfaces have applied to them electricalpotentials that serve to create an electrical field in the writingtablet. However, extreme nonlinearities are often developed within thefield which tend to reduce considerably the usable area of the writingsurface.

Accordingly, another important object of the present invention is theelimination in a telewriting device of a resistive surface as thewriting tablet and with it the electrical potentials required to createan electric field.

These objects and others are realized in this invention by departingfrom the usual type of telewriting device. Specically, the inventionutilizes a conventional cathode ray tube, whose face forms a writingtablet, and a special stylus that contains a photosensitive element,eg., a photocell, at its tip. As the stylus is moved in front of theface of the cathode ray tube in forming cursive script, electricalsignals obtained from the photocell, via a ilexible conductive lead, forexample, are operated upon and fed back to the beam dellection plates ofthe tube, and the electron beam in the tube is made to track the movingstylus. Since the beam tracks the stylus, the feedback signals areindicative of the X and Y co-ordinates of the stylus upon the face ofthe tube, and they may be transmitted to any remote location to indicatethese coordinates.

To elaborate further upon the principles of this invention, the trackingof the stylus by the electron beam is accomplished by dellecting thebeam in 4a circular scan of high rotational frequency and small radius.When 3,089,918 Patented May 14, 1963 the circular locus of the beam iscentered at a point on the face of the tube that lies opposite thecenter of the photocell contained in the tip of the stylus, the outputfrom the photocell is essentially constant, i.e., it is a steady statepotential. Decentering of the beam, on the other hand, produces avarying output potential rfrom the photocell of a frequency equal to thefrequency of the circular scan of the beam. The products of this signaland each of the signals applied to the horizontal and verticaldeilection plates of the cathode ray tube to produce the circularrotation of the beam are related to` the degree of decentering in thehorizontal and vertical deflection directions, respectively.Accordingly, the output potential from the photocell is applied to apair of multipliers to produce two error signals. These error signalsare separately integrated to produce a pair of signals which, whenapplied to the deilection plates of the cathode ray tube, position thebeam adjacent the tip of the moving stylus and which are indicative ofthe position of the stylus. In the present invention, the signals -fromthe integrators are added, respectively, to the signals that create thecircular scan of the beam, and the summed signals are applied to thedeflection plates of the `cathode ray tube =both to create the circularlocus of the beam and to center it on a point upon the Iface of the tubeopposite the tip of the stylus.

As an additional feature of the invention, an alternative scanning modeis institut-ed should the beam become decentered beyond a certainpredetermined amount. Decentering may be detected by noting themagnitude of the photocell output and generating an out-of-contactsignal whenever the output falls below a predetermined threshold. In theIalternative mode, the electron beam is made to scan the face of thecathode ray tube in normal line sweep and frame sweep fashion until thebeam again commences to track the photocell. It may also be noted thatthe above-mentioned outer-contact signal is generated whenever thestylus is withdrawn from the face of the tube. Thus, an indication isgiven of the formation Vof discontinuous traces such `as, for example,the formation of individual alphabetic characters or individual words.

In another embodiment of the present invention, a special cathode raytube is employed as the Writing tablet. In this arrangement the cathoderay tube has a specially constructed face comprising, for example, aresistive film deposited on a thin dielectric layer. The stylus` in thiscase is composed entirely of a conductive material that is groundedeither directly, by the use of a flexible electrical conductor, orindirectly utilizing the self-capacity of the operator using the stylus.The electron beam is scanned in a circular locus, as described before,and is also intensity modulated. When the beam impinges upon a portionof the resistive lm that is opposite the conductive stylus, the stylusshunts the majority or the varying beam current to ground. By detectingthe beam current in the resistive lm and producing a pair of signalsproportional to the product of the envelope of the current and thepotentials supplied to the `deflection plates that create the circularscan, the stylus may be tracked `and its X and Y co-ordinates upon thewriting tablet noted in a fashion similar to that described above.

Although the use of cathode ray tubes as writing tablets in telewritingdevices is known, the present invention departs greatly from theprinciples of operation employed by such devices of die prior art. Forexample, in a telewriting device disclosed in Patent 2,553,245 grantedto L. Espenschied on May l5, 1951, a source of light contained in a`stylus is required to impinge upon a special photosensitive andelectron emissive tube face t create a beam of electrons. The beamcreated thereby is passed through a set of plates and the potentialsinduced therein are indicative of the position of the stylus. In thepresent invention, however, an independently-formed beam of electrons isemployed, and by utilizing the special tracking features outlined above,the beam is made to track exactly the moving stylus. With the precisetracking, potentials indicative of the exact position of the stylus areobtained from the deflection plates of the cathode ray tube.

The invention may be better comprehended by consulting the followingdetailed description of illustrative embodiments thereof taken inconjunction with the accompanying drawings in which:

FIG. l is a pictorial diagram, partially in block schematic form, of anembodiment of this invention utilizing a typical cathode ray tube as awriting tablet;

FIGS. 2 and 3 are graphical aids helpful in understanding the operationof the device depicted in FIG. l; and

FIG. 4 is a pictorial diagram, partially in block schematic form, of analternative embodiment of the invention utilizing a speciallyconstructed cathode ray tube as the writing tablet.

Referring to FIG. l, a typical cathode ray tube is illustrated having abeam producing electrode 11, an intensity control grid 12, and sets ofvertical deflection plates 13 and horizontal deflection plates 14. Thesignals responsible for the dellcction of the beam within the tube areapplied to the deection plates 13 and 14 through the conductors 53 and54. The beam of electrons is made to assume a circular scan of highrotational frequency and small radius by applying, for example, a signalof cosine wave form to the horizontal deflection plates and a signal ofsine wave form to the vertical deection plates. The apparatus forapplying such signals to the deflection plates will be described in moredetail hereinafter.

A stylus is supplied for forming cursive script on the face of the tube.Conveniently, the tip of the stylus contains a lens 22 which images upona photocell 21, contained within the stylus housing, the portion of theface of the tube in front of the stylus. When the circular locus of thebeam upon the face of the tube is such that its image is centered on thesurface of the photocell, the output from the photocell is essentially asteady state potential. However, when the image of the circular locus ofthe beam is not centered on the photocell, a sinusoidally varyingpotential is generated by the photocell of a fundamental frequency equalto the frequency of rotation of the beam. A better understanding of thetheory behind the invention may be obtained at this point by consultingFIG. 2.

In FIG. 2 the center of the photocell, indicated at point Cp, may beconsidered to lie upon the face of the cathode ray tube. The electronbcams counterclockwise circular locus, of radius r, is shown with itscenter at CL, the latter comprising the point of intersection of the twoco-ordinates that represent vertical deflection and horizontaldeflection. As pointed out above, the circular motion of the beam iscreated by applying a potential of cosine wave form, e.g., a cos wt, tothe horizontal deflection plates and a potential of sine wave form,e.g., a sin wt, to the vertical detiection plates.

In the gure, the beam is shown in a decentered position as compared withthe photocell, the amount of decentering being equal to the distance sbetween the points CP and CL. The line segment of length s joining thesetwo points forms an angle cp with the horizontal deflection coordinate.Thus, s cos rp is the amount of decentering of the beam in thehorizontal deection direction, while s sin :p is the amount ofdecentering in the vertical detiection direction. ln order to reduce thedistance s to zero, a potential related to s ccs o must be applied tothe horizontal deflection plates of the cathode ray tube, while apotention related to s sin rp must be applied to the vertical deilectionplates. The present invention derives such potentials from thephotocellrs output signal, ep, in the following fashion.

Still referring to FIG. 2, the instantaneous photocell potential, ep, isrelated to the distance between the position of the beam at a giveninstant and the center of the photocell at point Cp. The beams angularvelocity w is chosen to be sufficiently great in relation to the rate atwhich the stylus is moved across the face of the cathode ray tube thatthe point CP may be considered fixed during several complete rotationsof the beam around the point CL. It may be seen that ep is greatest whenthe beam is in position d. As the beam moves around to position e on itscircular locus, the photocell potential gradually decreases to a minimumvalue; as the beam moves from e back to d, ep increases from its minimumvalue back to its maximum value. Thus ep varies with time in anapproximate sinusoidal fashion about some positive potential K, at afrequency ar equal to the frequency of rotation of the beam, and with amaximum value at those times when the beam is in position d. Thus, forexample, if the beam commences its counterclockwise circular locus atpoint c at time t=0 and is at point d at time the potential ep may beexpressed in the following form:

where K, b, p, q z are all related to the distance s. Only the term bcos (wt-go), however, need be considered in this analysis, sincesubsequent multiplication and integration operations upon the photocellsignal cause the contributions to the error signal from the constantterm K and the higher harmonics to be zero.

FIG. 3 depicts the variation in the value of b for various amounts ofdecentering s. Positive values of s indicate that, with regard to FIG.2, the center of the photocell lies to the right of the center of thecircular locus of the beam. Similarly, negative values of .s indicatethat the center of the photocell lies to the left of the center of thebeams locus. It may be noted, if the response of the photocell ismonotonieally related to the distance from the beam, when the center ofthe photocell is circumscribed by the circular locus of the beam, i.e.,when the absolute Value of s is less than r, b varies monotonically withs. When the center of the photocell is not circumscribed by the locus ofthe beam, i.e., when the absolute value of s is greater than r, themonotonie relationship between s and b no longer exists. However, thealgebraic signs of b ands are always the same whether or not the centerof the photocell is circumscribed by the locus of the beam. Since b isrelated to s as shown in FIG. 3, b cos p and b sin p are related in thesame fashion, respectively, to s cos (p and s sin rp, the lattercomprising the components of decentering in the horizontal and verticaldeflection directions. Accordingly, the apparatus of FIG: 1 resolves thepotential ep into two components proportional to b cos p and b sin cp,and these potentials are utilized by virtue of their relationships to scos p and s sin rp, to center the locus of the beam on the photocell.

Referring once again to FIG. l, the signal developed by the photocell,the fundamental component of which is given as b cos (wt-q1), is coupledto a tracking unit 30 by means of lead 23. Conveniently, lead 23 is aexible electrical conductor which neither fetters the free movement ofthe stylus nor hampers the writing style of a person using the device.An amplifier 31 may be utilized, if required, to increase the magnitudeof the photocell signal.

The signal from the amplifier 31 is applied to two multipliers 37 and 38and also to a peak rectifier 32. Two signals from sources 36 and 39, acos wt and w sin wt, respectively, are also applied to the multipliers37 and 38. Since the fundamental component of the amplified signal fromthe photocell 21 may be designated as b cos (tv-ga), the output frommultiplier 37, i.e.,

(a cos wt) (b cos [wz-qq) may be expressed as:

ab ab cos ga-l-(eos 2wt)( eos gi-Hsin 2m) sm ,p

Similarly, the output from multiplier 38, i.e.,

(a sin wt)(b cos [wt-(p1) may be expressed as:

yb sin qa-l-(sin 2wt)(a-2b eos gi-(cos 2wt) g2 sin o) In the abovelmultiplications, the constant term K and the higher order harmoniccomponents of the photocell signal have been neglected, since theyproduce product terms that are sine and cosine functions of time, and,such functions are filtered out by subsequent integration operations.

As shown by Equations 2 and 3, each multiplication produces a componentof voltage that varies only with p. In Equation 2, that component isab/Z cos rp; in Equation 3 it is ab/ 2 sin go. From the prior analysis,it may be seen that these components of voltage are related to thedegree of decentering, in the horizontal and vertical deflectiondirections, respectively, of the circular locus of the beam from thecenter of the photocell. Accordingly, to isolate these components, thesignals from multipliers 37 and 38 are applied to the summingintegrators 44 and 45, respectively. The summing integrators depictedare of conventional form, for example, as described in Korn and Korn,Electronic Analog Computers, 143-45, 288-92 (1st ed., 1952). Each of theintegrators provides an -output potential that is proportional to theintegral of the sum of the inputs applied thereto. For the present, itwill be assumed that the other inputs to the summing integrators, i.e.,the input from pulse gate 41 and the input from the scale of n frequencydivider 42, are each zero, since they apply to an alternate scanningmode to be explained later. Summing integrators 44 and 45, are providedwith a reset mechanism similar to the reset apparatus as shown in theKorn and Korn reference, supra. Conveniently, each may comprise -a relayenergized by a signal from its reset lead, 46 or 47. Upon energization,the relay connects the output of its associated summing integrator to acommon reference potential, e.g., ground. Since, however, the resettingoperations pertain to the alternate scanning mode just referred to, nosignals are present on the reset leads, and accordingly, the only activeleads to integrators 44 and 45 are from multipliers 37 and 38,respectively.

Because the integrals of terms of the form sin nwt and cos not are zeroover a time long as compared with 21r/w, the integrated signals frommultipliers 37 and 38 represent the integrals of only the terms :zb/2cos qb and zb/2 sin go, respectively. Thus, the error potentials ab/ 2cos go and zb/2 sin p are integrated themselves to provide a pair ofsignals which, when applied to the deflection plates of the lcathode raytube, result in the circular locus of the beam becoming centered on `apoint upon the face of the tube opposite the tip of the stylus. In thisrespect it should be noted that integration of the signals zb/2 cos toand zb/2 sin p always produces deiiection signals of the proper polarityto reduce the error s. This may be seen from the fact that, since thedeflection voltages are proportioned to the integrals of the terms alb/2cos p and :zb/2 sin go, the terms themselves represent the rate ofchange of the beams deflection and, therefore, the velocity of thecenter of the rotating beam. From FIG. 3 it may -be seen that the`algebraic sign of b is always the same as that of s. Thus, as the beambecomes centered, i.e., as s approaches zero, the velocity of thecenter, CL, of the circular beam locus is always in the proper directionindependent of whether or not the beam circumscribes the stylus. If,however, the Isignals b cos p and b sin p are derived by the use of lowpass filters and applied directly, Without integration, to thedeliection plates of the cathode ray tube, the rate of change of theseterms would represent the velocity of the center of the locus of thebeam. In this case it is apparent from FIG. 3 that, as s approacheszero, the rate of change of b with respect to time changes algebraicsign. The change in sign occurs at s=r; `and suc-h a change in signindicates a change in the direction of the movement of the center of thelocus of the beam. Thus, without integration, operation of the inventionis readily achieved in the range of s where b and s vary monotonicallywith each other, i.e., in the range where the circular locus of the beamcircumscribes the center of the photocell.

Referring again to FIG. l, the signals from integrators 44 and 45 4arepassed to the adders 34 and 35 where they have added to them,respectively, the signals a cos wt and a sin wt. These last-namedsignals are added at this point to provide the sinusoidal signals thatproduce the circular rotation of the beam. The signal from each of theadders -is thus `composed of two components; first a sinusoidalcomponent that produces circular rotation of the beam, and second acomponent that positions the beam opposite the tip of the moving stylus.Accordingly, these signals are applied directly to the deiiection platesy13 and 1-4 of the cathode ray tube by leads 53 and 54, respectively.

- Since the beam tracks the movements of the stylus, the signals fromthe integrators 44 and 45 are indicative of the X and Y co-ordinates ofthe stylus upon the plane formed by the face of the cathode ray tube 10.These signals are obtained from the leads 51 and 52 and are applied to autilization device 55 Where they may be utilized in any fashion, c g.,to effect a reproduction, at a local monitor or at a remote point, ofthe graphic material then being written on the face of the tube 10.

The present invention contemplates an alternative scanning mode thateffects a search scan of the face of the vtube lby the beam whenever thebeam is not tracking the stylus. Such a scan, which may be, for example,a typical line by line sweep of the entire `faoe of the tube by thebeam, continues until the beam once again tracks the stylus. Thisscanning mode is useful to permit automatic orientation of the ybeamnear the stylus, so that tracking may commence, for example, when thestylus is returned to the face of the tube in a writing position afterit has been withdrawn therefrom. The alternative scanning mode iseffected in the following fashion.

The output from amplifier 31 is passed through a peak rectifier 32 whichdevelops a signal that is indicative of the peak output potentialproduced by the photocell 21. The signal from rectifier 32 is suppliedto one input of a comparator 33, the other input of which is energizedby a reference potential, for example, a fixed potential designatedVRef.. Whenever the signal from the peak rectifier falls below thereference potential, the output potential of the comparator changesfrom, for example, 0 volts to some positive value. Thus, whenever theoutput of the photocell falls below some predetermined magnitude, whichindicates that the beam is neither tracking the stylus nor in closeproximity thereto, a signal is supplied by comparator 33. The signalfrom the comparator energizes pulse gate `41 which passes a series ofpulses that are formed, for example, from the output of the sine l Wavegenerator 39 by the pulse former 40. Accordingly,

are added to the signal from multiplier 37, and the sum is integrated.However, since the photocell signal is extremely small at the timeduring which tracking is not taking place, the output of multiplier 37is virtually zero. Thus, the pulses from gate 41 are alone integrated toproduce an output potential from integrator 44 that increases with timeas a series of step voltages.

After each series of n pulses from the pulse gate 41, a scale of nfrequency divider 42 produces an output pulse which serves to reset thesumming integrator 44 and which is applied to one input of the summingintegrator 45. Summing integrator 45 integrates the series of -pulsesfrom the scale of lz frequency divider 4Z in a fashion similar to thatof integrator 44. After each series of m pulses from the frequencydivider 42, the scale of m frequency divider 43 produces a pulse whichresets the summing integrator 45. The net effect of the integrations bythe summing integrators 44 and 45 is to produce outputs that resemblesawtooth functions, although each sawtooth is composed of a series ofstep voltages. The output from the summing integrator 44 thus provides asawtooth output potential which builds up during every n pulse frompulse gate 41 and which is applied through the adder 34 to thehorizontal deflection plates 14 of the cathode ray tube. This in effectcauses the beam within the tube to sweep the face of the tube in normalline sweep fashion. Similarly, the output from the summing integrator45, which also is of a sawtooth wave form that builds up during every ntimes m pulses from pulse gate 41, is applied through the adder 35 tothe vertical deflection plates 13 of the cathode ray tube. In thisfashion, the beam is made to sweep across the face' of the tube intypical frame sweep fashion in the vertical direction.

Since the line sweep and frame sweep scanning raster may commence at anypoint upon the face of the cathode ray tube 10, the sawtooth wavesshould be of sufficient magnitude to ensure that the ybeam scans an areaof at least twice that of the face of the tube. In this fashion, theline sweep and frame sweep scanning raster effectively encompasses theentire face of the tube, regardless of the potentials previouslysupplied by the integrators.

Alternatively, the resetting of the integrators may be effected by meansof deflection voltage comparisons with predetermined voltage Valuescorresponding to maximum required horizontal and vertical deflections,respectlvely. By utilizing such comparisons and eliminating both thescale of n divider 42 and the scale of m divider 43, when thehorizont-al deflection voltage reaches its maximum value, the summingintegrator 44 is reset and a. pulse is applied to the summing integrator45. At maximum vertical deflection, the vertical integrator is reset. Inthis alternate arrangement the double-amplitude deflection voltages arenot needed.

During the search scan, the beam sweeps across the entire face of thetube 10 in typical line sweep and frame sweep fashion until the beam isin proximity to the tip of the stylus 20. At this time, the `amplifiedand peak rectified .potential from the photocell 21 is slightly greaterthan the reference potential VRef that is applied to comparator 33. Theoutput potential of comparator 33 thus changes from the above-mentionedpositive value to volts, and the pulse gate 41 is no longer energized.Thus, the summing integrators 44 and 4S no longer have applied theretothe voltage pulses from gate 41 and frequency divider 42, respectively.However, since the beam in the cathode ray tube is now adjacent the tipof the stylus 20, the normal tracking mode takes over by virtue of theerror signals developed by multipliers 37 and 38, and the beam onceagain tracks the stylus. `It should be noted that by adding the errorsignal from one of the multipliers to the associated pulse signal forthe search scan and applying the sum to one integrator, the embodimentdepicted can switch smoothly and permanently from the searching mode tothe tracking mode. If a common integrator was not utilized, such atransition most likely could not take place, since it would be merechance that the integrated multiplier signal alone would be equal to thesearch signal when the beam was opposite the stylus. Thus, as the device-switched back from the searching to the tracking mode, the beam wouldbe deflected away from the photocell, which would cause the searchingmode to start once again. The present arrangement overcomes thisproblem.

The signal from the comparator 33 is also used to generate anout-of-contact signal, as from the lead 50, which is applied to theutilization device 55. ln this fashion, whenever the electron beam inthe tube 10 assumes a search raster and is sweeping the face of the tubein line sweep and frame sweep fashion, a signal is generated. Thissignal is present whenever the stylus is either withdrawn from the faceof the tube or whenever the beam has become decentered from thephotocell by a predetermined amount. Such an out-of-contact signal maybe utilized to indicate that the X and Y signals from leads 51 and 52are not indicative of information pertaining to the tracing, upon theface of the cathode ray tube, of an alphabetic character, a numeral, orother graphic material.

In the embodiment illustrated in FIG. l, no mechanical restraints areplaced upon the stylus. A conducting ilexible lead 23, which connectsthe photocell 21 and the amplifier 31, is the only form of coupling thatjoins the photocell Wtih the rest of the circuitry. It may beappreciated that this does not hinder or cramp a writing style to anyappreciable degree. However, alternative arrangements may be employed tolessen even further the restraint placed upon the movement of thestylus, such as, for example, a radio transmitter contained within thestylus to transmit a signal indicative of the photocell potential to theamplifier 31 and thence on to the rest of the circuitry of FIG. l.

Turning now to FIG. 4, there is depicted another embodiment of thepresent invention. In this case, a special cathode ray tube 10 isemployed that is similar in all respects to conventional cathode raytubes except that it has a specially constructed face 17. This facecomprises an outside dielectric layer 16 which conveniently may be ofglass. Deposited upon the inside of the layer is a thin resistive film15. An electrical connection is made by the lead 19 to the thinresistive film at a point 18 on the inside 'of the face of the cathoderay tube. Lead 19 is connected to a demodulator 24, the output of whichis connected to a tracking unit 3i), corresponding to the tracking unit30 of FIG. 1.

A conductive stylus 20 is employed `in this embodiment in place of thestylus containing the photocell as depicted in FIG. l. The stylus 20 maybe grounded directly, for example, by the use of a flexible conductivelead connected ybetween yit and ground, or it may be grounded indirectlythrough the self-capacity of the operator writing with the stylus.

The beam in the cathode ray tube, produced by the electrode 1'1 that iisconnected to a suitable energizing potential, not shown, is given acircular scan of high rotational frequency and small radius, as in theembodiment of FIG. 1. In addition, the beam is intensity modulated by aradio frequency signal from the source 9 that is applied to the beamintensity control grid 12. intensity modulation of the beam is provided,since a varying signal is yrequired to pass current from the resistivefilm 15 to :the metal stylus 24), both of which may be considered tocomprise opposite plates of a capacitor. When the electron beam impingesupon the resistive film 15 at a point opposite the tip of the metalstylus 20, a majority of the varying beam current passes through the lmto the stylus and thence to ground. On the other hand, when the beamimpinges the resistive vfilm at a point that is not opposite the tip ofthe metal stylus, a majority of the varying beam current flows withinthe resistive film to the demodulator 24. Accordingly, by examining thecurrent flowing in the resistive film, one may obtain -an indication ofthe flow of current to the metal stylus.

It thus maybe seen that when the locus of the circular scan of the beamis opposite the center of the tip of the metal stylus, the demodulatorhas applied -toit a signal that is small and whose envelope isnon-varying. However, When decentering of the beam occurs, i.e., whenthe beam is not spinning around an area directly opposite the center ofthe tip of the metal stylus, the demodulator has applied to its a signalof radio `frequency containing a varying or modulated envelope. Theifrequency of the envelope -Will be equal to the `frequency of rotationof the beam within the cathode ray tube. As in the embodiment of FIG. l,the products of this envelope and the potentials applied to the platesof the cathode ray tube that cause the circular scan are indicativeofthe magnitude and direction of `decentering of the beam. Accordingly,the signal from the resistive film is demodulated and applied to atracking unit 30 which is virtually the same tracking unit as depictedin FIG. l. The only differences in the units are that, yfor the presentembodiment, the inputs to multipliers 37 and 38 from oscillators 36 and39 are -a cos wt and a sin wt, respectively. Further, the comparator 33is altered to energize the pulse gate 41 when the amplified and peakrectified signal from demodulator 24 is greater than VREL. These changesare required since the fundamental component of the signal from theresistive film may be expressed as -b cos (wt-o), Whereas thelfundamental component of the signal from the photocell is b cos (wt-p)and also because decentering of the beam results in an increase in thesignal from the resistive film, whereas decentering causes a decrease inthe photocell signal. With these changes, the apparatus operates as thatof FIG. l, and the beam tracks the movements of the metal stylus. Thus,three signals are obtained, namely, an out-of-contact signal from lead Sindicating when the stylus is being tracked, and X and Y signals fromleads Si and 52 indicating the yco-ordinates of the stylus with respectto the writing tablet.

For this embodiment, a special stylus containing a photocell is notrequired. Any metal stylus Will suffice; an ordinary metallic `fountainpen or ballpoint pen may be used.

Although specific embodiments of the present invention have beendepicted, it is obvious that numerous additions `and substitutions maybe made without departing from the spirit and scope of the invention.Accordingly, the invention should be deemed limited only insofar as itis restrictively defined in the following claims.

What is claimed is:

l. Telewriting apparatus comprising a cathode ray tube, means -forproducing a beam of electrons within said tube, means for ideiiectingsaid beam in response to input signals, means -for supplying anoscillatory signal to said beam deflection means for rotating said beamabout a reference axis, a photocell, means for detecting `an output fromsaid photocell as it is moved in closed proximity to the Aface of saidcathode ray tube, means supplied with the output of said photocell andsaid oscilla-tory signal `for producing a potential corresponding to the`difference in phase therebetween `and of a magnitude dependent on thedistance between said photocell and the point of impingement of saidbeam on the face of said tube, means lfor integrating said potential,means for applying said integrated potential to said beam deflectionmeans for detlecting said beam so that said reference axis passesthrough said photocell, means for applying a signal to said beamdeection means to deflect said electron beam in a search raster acrossthe face of said tube Whenever the magnitude of said photocell output isless than a predetermined threshold, and means lfor utilizing theaforementioned integrated potential to indicate the X and Y co-ordinatesof said photocell.

2. Apparatus for transmitting electrical `signals indicative of theposition of a photocell adjacent the face of a cathode ray tube in whichsaid tube includes means for creating a beam of electrons and 4a firstmeans for deflecting said beam in a first direction and a second means`for deflecting said beam in a second direction; said apparatuscomprising means yfor applying a first signal to said first deflectionmeans and a second signal to said second deflection means, said firstand second signals exhibiting a phase difference suflicient to impart tosaid beam a rotational movement about a reference axis, a photocell,means for detecting an output signal from said photocell, means suppliedwith said photocell signal and said first signal for producing a firstpotential proportional to the component in said first direction of thedistance between the point of impingement of said beam upon said faceand said photocell, means supplied with said photocell signal and saidsecond signal for producing a second potential proportional to thecomponent in said second direction of the distance between the point ofimpingement of said beam upon said face and said photocell, means forproducing an out-of-contact signal Whenever said output signal from saidphotocell is less than a predetermined magnitude, means responsive tosaid out-of-contact signal for producing a first search signal, meansresponsive to said out-of-contact signal for producing a second searchsignal, said first search signal and said second search signal, whenseparately integrated and applied to said first and second deflectionmeans, respectively, imparting to said beam a movement that effects asearching raster covering the entire face of said tube, means for addingsaid first potential and said first search signal to produce a firstsummed signal, means for adding said second potential and said secondsearch signal to produce a second summed signal, means -for integratingsaid first summed signal, means for integrating said second summedsignal, -means for applying said integrated first summed signal to saidfirst deflection means, means for applying said integrated second summedsignal to said second deiiection means, and means for utilizing saidintegrated rst summed signal, said integrated second summed signal andsaid out-of-contact signal to indicate the movements of said photocell.

3. A telewriting device comprising a cathode ray tube, means forproducing a beam of electrons within said tube, a first deflection meanslresponsive to an input signal for deflecting said beam in a firstdirection, .a second deection means responsive to an input signal fordefiecting said beam in a second direction displaced 9() degrees fromSaid first direction, a stylus capable of being moved about the face ofsaid tube, a photosensitive element positioned in said stylus, means fordetecting an output signal from said photosensitive element, means formultiplying said output signal from said photosensitive element by asignal of cosine wave form to produce a first product signal, means formultiplying said output signal from said photosensitive element by asignal of sine Wave form to produce a second product signal, means forproducing an out-of-contact signal whenever the magnitude of said outputsignal from said photosensitive element is less than a predeterminedvalue, means responsive to said out-0fcontact signal for producing asignal of a first pulse Wave form, means responsive to saidout-of-contact signal for producing a signal of a second pulse Waveform, means for adding said first product signal and said first pulsesignal to produce a rst summed signal, means for adding said secondproduct signal and said second pulse signal to produce a second summedsignal, first means for integrating said first summed signal, secondmeans for integrating said second summed signal, means for resetting toa first reference value the output of said lfirst integrating meansafter each series of n pulses from said first pulse Wave form means,means for resetting to a second reference value the output of saidsecond integrating means after each series of m pulses from said secondpulse Wave form means, means for adding said integrated first summedsignal and said signal of cosine Wave form to produce a first controlsignal, means for adding said integrated second summed signal and saidsignal ot` sine wave form to produce a second control signal, means forapplying said rst control signal to said first deflection means, meansfor applying said second control signal to said second deflection means,and means for transmitting said integrated first summed signal, saidintegrated second summed signal, and said out-of-contact signal to anylocation thereby to provide signals indicative of the movement of saidstylus.

4. A device for the transmission of graphic information comprising acathode ray tube, the face of said cathode ray tube comprising aconductive film placed upon a layer of dielectric material, means forproducing a beam of electrons within said tube to impinge upon a portionof said conductive film thereby to produce a flow of current therein,means for modulating the intensity of said beam in accordance with anapplied signal to cause intensity variations of said current, controlmeans for deflecting said beam in response to an input signal, aconductive stylus coupled to a source of reference potential and capableof being freely manipulated adjacent said dielectric layer to therebyproduce an amplitude modulation envelope on the intensity variations ofsaid current, demodulating means responsive to the flow of current insaid conductive film for developing a signal Whose wave form follows theenvelope of said current flowing in said conductive film, meansresponsive to the signals from said demodulating means and to said inputsignal for supplying a control signal to said control means forpositioning said beam to impinge the portion of said face of said tubeopposite said stylus, and means for utilizing said control signal toindicate the movement of said conductive stylus.

5. A device as recited in claim 4 further comprising means responsive tosaid signal from said demodulating means for supplying a varyingpotential to said control means to cause said beam to be deilected in aSearch raster across the face of said tube whenever the magnitude ofsaid demodulated signal is greater than a predetermined threshold.

6. Telewriting apparatus comprising a cathode ray tube, the face of saidcathode ray tube comprising a thin resistive film and a layer ofdielectric material, means for producing a beam of electrons within saidtube, said beam producing a current flow in said resistive film uponimpingement therewith, means for modulating the intensity of said beamin accordance with an applied signal thereby causing intensityvariations in said current, means for deflecting said beam in responseto input signals, means for supplying an oscillatory signal -to saidbeam deflection means for oscillating said beam in a locus about areference axis, a source of reference potential, a metal stylus capableof being moved freely about the face of said tube adjacent saiddielectric layer and coupled to said source of reference potential,means for detecting the flow of current in said resistive lrn as saidstylus is moved in close proximity to said face of said tube, saidstylus producing an amplitude modulation envelope on the intensityvariations of said current when proximate to a point on said face whichis closer to one part of the locus of said beam than to another part ofthe locus, demodulating means for developing a signal whos-e wave formis the envelope of said current flowing in said resistive film, meanssupplied with said last-named signal and said oscillatory signal forproducing a potential corresponding to the difference in phasetherebetween, means for integrating said potential, means for applyingsaid integrated potential to said beam deflection means for deilectingsaid beam so that said reference axis passes through said stylus, meansfor applying a signal to said beam deflection means deflect saidelectron beam in a search raster across said face of said tube wheneverthe magnitude of said demodulated signal is greater than a predeterminedthreshold, and means for utilizing said integrated potential to indicatethe position of said stylus adjacent said face of said tube.

7. Apparatus for transmitting electrical signals indicative of theposition of a conductive stylus adjacent the face of a cathode ray tubein which said tube is characterized by a face ycomprising a resistivefilm, said tube being further characterized by means for creating a beamof electrons, said beam producing a current flow in said resistive filmupon impingement therewith, means for cyclically varying the intensityof said beam, a first means for deflecting said beam in a firstdirection, and a second means for deflecting said beam in a seconddirection; said apparatus comprising means for applying a first signalto said first defiection means and a second signal to said seconddeflection means, said first and second signals exhibiting a phasedifference sufiicient to impart to said beam an oscillatory movementabout a reference axis, Wave form developing means for developing asignal whose Wave form is lthe envelope of the current fiowing in saidresistive film, means supplied `with the signal from said wave formdeveloping means and said first signal for producing a first potentialproportional to the distance in said first direction between the pointof impingement of said beam upon said face and said stylus, meanssupplied with said signal from said wave form developing means and saidsecond signal for producing a second potential proportional to thedistance in said second direction between the point of impingement ofsaid beam upon said face and said stylus, means for applying said first'potential to said first deflection means, means for applying said secondpotential to said second deflection means, and means for utilizing saidfirst potential and said second potential to indicate at any locationIthe movements of said stylus.

8. Apparatus for transmitting electrical signals indicative of theposition of a conductive stylus adjacent the face of a cathode ray tubein which said tube is characterized by a face comprising a resistivefilm, said tube being further characterized by means for creating a beamof electrons, said beam producing a current flow in said resistive filmupon impingement therewith, means for cyclically varying the intensityof said beam thereby producing cyclical variations of said current, afirst means for deflecting said beam in a first direction, and a secondmeans for deflecting said beam in a second direction; said apparatuscomprising means for applying a first signal to said `first deflectionmeans and a second signal to said second deflection means, said firstand second signals exhibiting a phase difference sufficient to impart tosaid beam an oscillatory locus about a reference axis, said stylusproducing an amplitude modulated envelope wave form on the cyclicalvariations of said current whenever any part of the locus of said beamon said resistive film is closer than any other part of the locus to apoint at which said stylus is proximate to the resistive film, wave formdeveloping means for developing a signal whose wave lform is theenvelope Wave form of the current flowing 1n said resistive film, meanssupplied with the signal from said Wave form developing means and saidfirst signal for producing a first potential proportional `to thedistance in said first direction between the point of impingement ofsaid beam upon said face and said stylus, means supplied With saidsignal from said wave form developing means and said second signal forproducing a second potential proportional .to ythe distance in saidsecond direction between the point of impingement of said beam upon saidface and said stylus, means for integrating said first potential, meansfor integrating said second potential, means for applying saidintegrated first potential to said first deflection means, means forapplying said integrated second potential to said second deflectionmeans, and

13 means for utilizing said integrated first potential and saidintegrated second potential to indicate the movements of said stylus.

9. Apparatus for transmitting electrical signals indicative of theposition of a conductive stylus connected to a source of referencepotential as said stylus is moved in close proximity to the face of acathode ray tube, said tube being characterized by a face comprising aresistive iilm and said tube including means for creating a beam ofelectrons, said beam producing a current iiow in said resistive iilmupon impingement therewith, means for cyclically varying the intensityof said beam thereby producing cyclical variations of said current, aiirst means for deecting said beam in a iirst direction, and a secondmeans for dedecting said beam in a second direction; said apparatuscomprising means for applying a iirst signal to said tirst deflectionmeans and a second signal to said second deection means, said iirst andsaid second signals exhibiting a phase difference suflicient to impartto said beam an oscillatory locus about a reference axis, said stylusproducing an amplitude modulated envelope wave form on the cyclicalvariations of said current whenever any part of the locus of said beamon said resistive iilm is closer than any other part of the locus ofsaid beam to a point at which said stylus is proximate to the resistiveiilm, wave form developing means for developing a signal whose ywaveform characterizes the envelope wave form of the current flowing in saidresistive iilm, means supplied with the signal from said lwave formdeveloping means and said iirst signal for producing a iirst potentialproportional to the distance in said irst direction between the point ofimpingement of said beam upon said face and said stylus, means suppliedwith said signal from said wave form developing means and said secondsignal for producing a second potential proportional to the distance insaid second direction `between the point of impingemen-t of said beamupon said face and said stylus, means for producing an out-of-contactsignal whenever the magnitude of said signal from said wave formdeveloping means is greater .than a predetermined magnitude, meansrespon sive to said out-of-contact signal for producing a iirst searchsignal, means responsive to said contact signal for producing a secondsearch signal, said iirst search signal and said second search signalimparting to said beam a searching pattern which covers the entire faceof said tube when said signals are each integrated and applied to saidrst deection means and said second deection means, respectively, meansfor adding said tirst potential and said iirst search signal to producea rst summed signal, means for adding said second potential and saidsecond search signal to produce a second summed signal, means forintegrating said first summed signal, means for integrating said secondsummed signal, means for applying said integrated rst summed signal tosaid iirst deliection means, means for applying said integrated secondsummed signal to said second deection means, and means for utilizingsaid integrated iirst potential, said integrated second potential, andsaid out-ofcontact signal Ito indicate the movements of said stylus.

l0, A telewriting device comprising a cathode ray tube having a facecomposed of a layer of dielectric material upon which has been depositedupon one side thereof a thin resistive iilm, means for producing a beamof electrons within said tube, said beam producing a current flow insaid resistive film upon impingement therewith, means for cyclicallyvarying the intensity of said beam thereby producing cyclical variationsof said current, iirst and second deection means, means for applying apotential of cosine wave form to said iirst deflection means and apotential of sine Wave form to said second deiiection means for rotatingsaid beam in a circle on said resistive iilm, a source of groundpotential, a grounded conductive stylus capable of being moved freelyabout the face of said tube adjacent said dielectric layer, said stylusproducing an amplitude modulated envelope wave form on the cyclicalrvariations of said current when a part of said circle is closer thananother part of said circle to a point a-t which said stylus isproximate to said dielectric material, means for producing a signalproportional to the amount of current in said resistive film, means fordetecting the envelope tvave form of said proportional signal, means formultiplying said envelope Wave form of said signal by the potential ofcosine lwave form to produce a tirst product potential, means formultiplying said envelope wave form of said signal by the potential ofsine Wave form to produce a second product potential, means forproducing an out-of-contact signal Whenever the magnitude of saidenvelope wave form is greater than a predetermined magnitude, meansresponsive -to said outof-contact signal for producing a potential of aiirst pulse wave form, means responsive to said out-of-contact signalfor producing a potential of a second pulse Wave form, means for addingsaid iirst product potential and said rst pulse potential to produce afirst summed potential, means for adding said second product potentialand said second pulse potential to produce a second summed potential,first means for integrating said iirst summed potential, second meansfor integrating said second summed potential, means for resetting to afirst reference value the output of said iirst integrating means aftereach series of n pulses from said iirst pulse Wave form means, means forresetting to a second reference value the output of said secondintegrating means after each series of m pulses from said second pulseWave form means, means for adding said integrated iirst summed potentialand said potential of cosine 'wave form to produce a first controlsignal, means for adding said integrated second summed potential andsaid potential of sine Wave form to produce a second control signal,means for applying said first control signal to said tirst deflectionmeans, means for applying said second control signal to said seconddeflection means, and means for transmitting said integrated iirstsummed potential, said integrated second summed potential, and saidout-of-contact signal to any location thereby to provide signalsindicative of the movement of said stylus.

lfl. Telewriting apparatus comprising a cathode ray tube, means forproducing a beam of electrons within said tube, means for deflectingsaid beam in response to input signals, means for supplying anoscillatory signal to said beam deflection means for rotating said beamabout a reference axis, a photosensitive element, means for detecting anoutput from said photosensitive element as it is moved in closeproximity to the face of said cathode ray tube, means supplied with Itheoutput of said photosensitive element and said oscillatory signal forproducing an error potential corresponding to the diierence in phasetherebetween, means for applying said error potential to said beamldeflection means for deflecting said beam so that said reference axispasses through said photosensitive element, means responsive to theoutput of said photosensitive element for supplying a varying potentialto said deection means to deilect said beam in a search raster acrossthe face of said tube `whenever the magnitude of said photosensitiveelement output is less lthan a predetermined threshold, and means forutilizing said error potential to indicate the X and Y coordinates ofsaid photosensitive element.

l2. A device for the transmission of graphic information comprising acathode ray tube, means for producing a beam of electrons within saidtube, control means for detlecting said beam in response to an inputsignal, means for supplying an oscillatory signal to said beamdeflection means for rotating said beam about a reference axis, stylusmeans positioned in a nonxed fashion adjacent the face of said tube,means responsive to the location of said stylus means relative to thelocation of said beam on the face of said tube for producing a controlcurrent,

means supplied with said control current and said oscillatory signal forproducing an error potential corresponding to the difference in phasetherebetween and of a magnitude dependent on the distance between saidstylus and the point of impingement of said beam on the face of saidtube, means for supplying said error potential to said control means toposition said beam upon the portion of said face of said tube oppositesaid stylus means, means responsive to said control current forsupplying a varying potential to said deieotion 'control means todellect said beam in a search raster across the face of said 15 tubewhenever the location of said stylus means relative to the location ofsaid beam on the face of said tube is greater than a predetermineddistance, and means for utilizing said error potential to provideinformation relevant to the movement of said stylus means.

References Cited in the le of this patent UNITED STATES PATENTS2,455,532 Sunstein Dec. 7, 1943 10 2,487,641 Denk Nov. 8, 1949 2,849,707White Aug. 26, 1958

1. TELEWRITING APPARATUS COMPRISING A CATHODE RAY TUBE MEANS FORPRODUCING A BEAM OF ELECTRONS WITHIN SAID TUBE, MEANS FOR DEFLECTINGSAID BEAM IN RESPONSE TO INPUT SIGNALS, MEANS FOR SUPPLYING ANOSCILLATORY SIGNAL TO SAID BEAM DEFLECTION MEANS FOR ROTATING SAID BEAMABOUT A REFERENCE AXIS, A PHOTOCELL, MEANS FOR DETECTING AN OUTPUT FROMSAID PHOTOCELL AS IT IS MOVED IN CLOSED PROXIMITY TO THE FACE OF SAIDCATHODE RAY TUBE, MEANS SUPPLIED WITH THE OUTPUT OF SAID PHOTOCELL ANDSAID OSCILLATORY SIGNAL FOR PRODUCING A POTENTIAL CORRESPONDING TO THEDIFFERENCE IN PHASE THEREBETWEEN AND OF A MAGNITUDE DEPENDENT ON THEDISTANCE BETWEEN SAID PHOTOCELL AND THE POINT OF IMPINGEMENT OF SAIDBEAM ON THE FACE OF SAID TUBE, MEANS FOR INTEGRATING SAID POTENTIAL,MEANS FOR APPLYING SAID INTEGRATED POTENTIAL TO SAID BEAM DEFLECTIONMEANS FOR DEFLECTING SAID BEAM SO THAT SAID REFERENCE AXIS PASSESTHROUGH SAID PHOTOCELL, MEANS FOR APPLYING A SIGNAL TO SAID BEAMDEFLECTION MEANS TO DEFLECT SAID ELECTRON BEAM IN A SEARCH RASTER